(a) Field of the Invention
The present invention relates to nickel powder and a conductive paste containing the same and more particularly to nickel powder and a conductive paste containing the same, which can control heat shrinkage while inhibiting the occurrence of any rapid oxidation and which permit the production of a thin, uniform internal electrode for a multilayer ceramic capacitor without being accompanied by any crack formation and delamination during firing.
(b) Description of the Prior Art
A multilayer ceramic capacitor comprises a plurality of ceramic dielectric layers and internal electrode layers, which are alternately laminated and united. In the production of the internal electrode for such a multilayer ceramic capacitor, it is common that metal fine powder as a material for internal electrodes is first formed into a paste thereof to give a conductive paste, followed by printing a green sheet of a ceramic dielectric with the resulting conductive paste, alternately putting, in layers, a plurality of these green sheets of the ceramic dielectric and the conductive paste layers, attaching the latter to the former using pressure to thus unify them, and then firing the resulting laminated assembly in a reducing atmosphere at a high temperature to thus firmly unify the ceramic dielectric layers and the internal electrodes.
As materials for such an internal electrode, there have conventionally been used, for instance, precious metals such as platinum, palladium and silver-palladium. However, there have recently been developed techniques in which base metals such as nickel are substituted for these precious metals and such techniques have gradually been advanced, for the purpose of reducing the production cost. When forming a conductive paste layer using a paste containing nickel powder and then firing the resulting paste layer to produce a thin and uniform internal electrode, however, problems such as crack formation and delamination arise. The occurrence of such crack formation and delamination would be caused due to, for instance, heat shrinkage of the conductive paste layer during firing.
Accordingly, there have conventionally been proposed a variety of powdery nickel products, which can solve the problem of heat shrinkage during firing.
Similarly, it is an object of the present invention to provide nickel powder as well as a conductive paste containing the nickel powder, which can control heat shrinkage while inhibiting the occurrence of any rapid oxidation and which accordingly, permit the production of a thin, uniform internal electrode for a multilayer ceramic capacitor without being accompanied by any crack formation and delamination during firing.
The inventors of this invention have conducted various studies to solve the foregoing problems associated with the conventional techniques, have found that the foregoing problems of rapid increase in the oxidation of the conductive paste layer and the heat shrinkage thereof during firing are solved by increasing the average particle density of the nickel powder to a level greater than a desired value and reducing the average diameter of the crystallites present in the particles to a level smaller than a predetermined value and that the resulting conductive paste permits gentle sintering, can ensure a uniform sintering speed and accordingly, permits the formation of a thin and uniform internal electrode for a multilayer ceramic capacitor without being accompanied by any crack formation and delamination, and have thus completed the present invention based on the foregoing findings.
According to an aspect of the present invention, there is provided nickel powder having an average particle size, as determined by the observation under a scanning electron microscope (SEM), of not more than 1 xcexcm, a particle density of not less than 8.0 g/cm3, and an average diameter of crystallites present in the nickel particles of not more than 550 xc3x85.
According to a second aspect of the present invention, there is provided a conductive paste, which comprises nickel powder having characteristic properties defined above.
The nickel powder of the present invention is particularly suitable for use in making an internal electrode for a multilayer ceramic capacitor or in the preparation of a conductive paste for a multilayer ceramic capacitor. Accordingly, the average particle size of the nickel powder of the present invention, as determined by the observation thereof with an SEM with a magnification of about xc3x9710000, is limited to a level of not more than 1 xcexcm, preferably 0.1 to 1 xcexcm and more preferably 0.2 to 0.8 xcexcm, while taking into consideration the foregoing applications of the nickel powder.
The meaning of the term xe2x80x9cparticle densityxe2x80x9d, used in this specification, is identical to that defined in JIS R 1600 (1993). More specifically, the term xe2x80x9cparticle densityxe2x80x9d is the density of particles, in which the closed spaces included in the particles are likewise regarded as parts of particles. If the particle density is reduced, the amount of void spaces present within particles correspondingly increases, while if the particle density increases, the amount of void spaces present within particles are correspondingly reduced. These void spaces expand when exposed to a high temperature during the high temperature firing step in the production of a multilayer ceramic capacitor and at least part thereof is destroyed to cause shrinkage of the particle. Thus, the presence of such void spaces would greatly affect the thermal properties, in particular, heat shrinkage of the conductive paste layer containing nickel powder. In the present invention, the particle density of the nickel powder is controlled to not less than 8.0 g/cm3, preferably not less than 8.3 g/cm3 and more preferably not less than 8.5 g/cm3. Increasing the particle density of the nickel powder can thus eliminate the problems of any crack formation and delamination.
Moreover, the average diameter of crystallites present in the nickel particles of nickel powder of the present invention is controlled to a level of not more than 550 xc3x85, preferably not more than 500 xc3x85 and more preferably not more than 300 xc3x85. A product obtained using a paste containing such nickel powder, for instance, a multilayer ceramic capacitor, is composed of a tidy sintered film and the sintering of the paste proceeds gently because the average diameter of the crystallites present in the nickel particles is small. In other words, the sintering of such a paste rapidly proceeds if the crystallites present in the nickel particles have a large diameter, but if the average diameter thereof is small, the crystallites present in the nickel particles are first sintered and then the sintering between the nickel particles gradually proceeds. Accordingly, the nickel powder-containing paste is gradually sintered at a uniform speed to thus form a tidy film and the paste hardly causes any crack formation and delamination.
As has been discussed above in detail, the nickel powder of the present invention has an average particle size as determined by the SEM observation thereof of not more than 1 xcexcm, a particle density of not less than 8.0 g/cm3, and an average diameter of crystallites present in the nickel particle of not more than 550 xc3x85. Accordingly, the conductive paste obtained using such nickel powder would permit the formation of a thin and uniform internal electrode for a product such as a multilayer ceramic capacitor, without causing any crack formation and delamination during firing.
The nickel powder of the present invention, which satisfies various requirements discussed above, is suitably used for preparing a conductive paste, in particular, a conductive paste for making a multilayer ceramic capacitor.
Therefore, the conductive paste according to the present invention comprises the foregoing nickel powder having excellent characteristic properties described above and is thus particularly suitable for use in making a thin and uniform internal electrode for a multilayer ceramic capacitor.
Now, we will hereunder explain a preferred method for preparing the nickel powder of the present invention.
The nickel powder of the present invention can be prepared by either a wet method or a dry method, but it is preferably prepared by a wet method. In such a wet method for preparing the nickel powder of the present invention, for example, nickel hydroxide prepared by reacting a nickel salt with an alkali or commercially available nickel hydroxide per se is reduced by bringing it into contact with a hydrazine type reducing agent at a temperature condition of not less than 55xc2x0 C., while the rate of nucleation and the rate of growth of fine nickel particles are stepwise controlled. Examples of such nickel salts are nickel sulfate, nickel nitrate and nickel halides such as nickel chloride. On the other hand, examples of such alkalis are sodium hydroxide, potassium hydroxide and calcium hydroxide. Examples of the foregoing hydrazine type reducing agents are hydrazine, hydrazine hydrate, hydrazine sulfate, hydrazine carbonate and hydrazine hydrochloride.
Regarding the temperature conditions for the reducing reaction of the nickel hydroxide, it is preferable to add a reducing agent to the reactant at a temperature of less than 50xc2x0 C. and then gradually raise the temperature to a temperature of not less than 55xc2x0 C. to proceed the reaction slowly. By using this process, a nickel powder can be obtained, which has a small average diameter of crystallites, a high particle density and a uniform average particle size.
In particular, the nickel powder having a particle density of not less than 8.0 g/cm3 and a uniform particle size as determined by the SEM observation can be prepared by adding an aqueous solution of hydrazine to nickel hydroxide obtained through a reaction of a nickel salt with sodium hydroxide or commercially available nickel hydroxide per se at a temperature of less than 50xc2x0 C. and then gradually raising the temperature at a heating speed of not more than 5xc2x0 C./min to a temperature of not less than 55xc2x0 C., preferably not less than 60xc2x0 C. to proceed the reduction slowly and the resulting nickel powder has a quite low content of the whole impurities derived from the starting materials.
The nickel powder of the present invention can be obtained in the form of monodispersed nickel powder, which is obtained by bringing nickel hydroxide into contact with a hydrazine type reducing agent at a temperature of not less than 55xc2x0 C. to thus reduce the hydroxide and then the resulting powdery product is subjected to a pulverization treatment. Such pulverization treatment usable herein are, for instance, high speed rotary collision-pulverization treatment in which nickel powder is pulverized by leading a rotary part, rotating at a high speed, of a pulverizer to collide with the powder; a medium-stirring pulverization treatment in which nickel powder is stirred with, for instance, beads to thus pulverize the same; a high hydraulic pressure pulverization treatment, which comprises colliding two streams of aqueous nickel powder slurries injected from different directions at a high hydraulic pressure to thus pulverize the nickel powder; and a jet-impact treatment and one can use, for instance, a high speed moving body-collision type air pulverizer, an impact type pulverizer, a cage mill, a medium-stirring type mill, axial-flow mill and a jet-colliding device.
Next, we will explain a preferred method for preparing a conductive paste according to the present invention, below in detail.
The conductive paste of the present invention is constituted by, for instance, the foregoing nickel powder of the present invention, a resin and a solvent. Optionally, it may further comprise a dispersant, a sintering-inhibitory agent or the like. More specifically, examples of such resins usable herein are cellulose derivatives such as ethyl cellulose, vinylic non-curable resins such as acrylic resins, polyvinyl butyral resins and polyvinyl alcohol, and thermosetting resins preferably used in combination with peroxides, such as epoxy resins and acrylic resins. Resins usable herein further include, for instance, UV curable resins, electron beam-curable resins such as epoxy acrylate resins, polybutadiene acrylate resins and urethane acrylate resins modified with acrylic acid or methacrylic acid, and unsaturated polyesters. In this connection, in case where the resin used is a UV curable resin, an optical initiator should be used and examples thereof include benzoin, acetophenone, benzyl, benzophenone and benzoin butyl ether. In addition, examples of such solvents usable herein are terpineol, tetralin, butyl carbitol and carbitol acetate, which may be used alone or in any combination. Moreover, this paste may if necessary comprise glass frits. The conductive paste of the present invention can be prepared by mixing and stirring the foregoing raw materials in a mixing device such as a ball mill or a three-roll mill.
The nickel powder of the present invention permits the control of any thermal shrinkage of the resulting sheet or film while inhibiting any rapid oxidation and as a result, the powder permits the formation of a thin and uniform internal electrode for a multilayer ceramic capacitor without causing any crack formation and delamination. Thus, the nickel powder of the present invention is suitably used for preparing a conductive paste, in particular, a conductive paste for a multilayer ceramic capacitor.
In addition, the conductive paste according to the present invention comprises nickel powder having the foregoing excellent characteristic properties and therefore, the paste is particularly suitably used in making a thin and uniform internal electrode for a multilayer ceramic capacitor.
The present invention will now be described below in detail with reference to the following working Examples and Comparative Examples.